Not including bonus damage from skill speed, Venomous Bite is 340 potency and Windbite is 360. Caustic Bite is 570 and Stormbite is 670. Quick Nock is 110 per target and Rain of Death is 100 per target. So with the weaker DoTs, if you have three targets, you should definitely put both dots up on everything.

It's a little harder to say with the stronger dots, since they don't tick much harder, just longer. This means you can't always count on the full 30s duration when dotting a lot of targets, whereas you could usually count on the 18s dots ticking completely. But it's worth putting up at least Stormbite on a few targets so you can get Ballad procs to reset Rain of Death. Those extra 100/e free oGCD AoEs are delicious.